Catalyst supported on magnesium dihalide in active form are well-known from the technical literature. They were disclosed the first time in U.S. Pat. Nos. 4,298,718 and 4,495,338.
The need for having available high-activity catalysts capable of producing polymers with controlled morphological characteristics is felt in the industrial practice.
Examples of catalysts with controlled morphology are described in U.S. Pat Nos. 3,953,414 and 4,399,054. In the latter patent the components are prepared by starting from spherical form adducts of MgCl.sub.2 with approximately 3 mols of alcohol. Prior to the reaction with TiCl.sub.4 the alcohol content is decreased down to 2.5-2 mols: in this manner components are obtained which exhibit a porosity, as measured with nitrogen, of from 0.3 to 0.4 cm.sup.3 /g and an average pore radius comprised between 15 and 20 .ANG..
Catalyst prepared from TiCl.sub.4 and MgCl.sub.2 in granular form, by spray-drying an alcohol solution of magnesium chloride and subsequently supporting the titanium compound, are described in patents EP-B-65700 and EP-B-243327. However, the polymer obtained with these catalysts does not exhibit morphological characteristics of interest. In particular the bulk density is not sufficiently high. Furthermore, the activity of the catalyst is rather low.
A method for increasing the activity of these catalysts is described in patent EP-A-281524. The catalysts are prepared by supporting titanium alcoholates onto an MgCl.sub.2 -ethanol adduct containing from 18 to 25% by weight of ethanol spherulized by spray-drying an ethanol solution thereof, and subsequent chemical treatment with Et.sub.2 AlCl or Et.sub.3 Al.sub.2 Cl.sub.3. The conditions for preparing the support are critical and have an influence on the morphological stability of the resulting polymer. Polymers in the form of heterogeneous powder are obtained, for example, when supports are used the alcohol content of which is not comprised within the range 18-25%, or when compounds different from Et.sub.2 AlCl or Et.sub.3 Al.sub.2 Cl.sub.3 are used. Furthermore, in order to obtain sufficiently high yields, the Ti content in the solid component is always greater than 8% by weight.
From European patent application EP-A-395083, high-activity catalysts for olefin polymerization are known which are capable of producing polymers in the form of particles of spheroidal shape endowed with satisfactory morphological properties, in particular high bulk density.
When these catalysts are used for the polymerization of ethylene to produce LLDPE or in general ethylene copolymers with other .alpha.-olefins, the distribution of the comonomer in the polymeric chain is far from optimum.
The solid components of the catalysts described in EP-A-395083 are characterized by a high porosity (as measured by mercury method) and by a distribution of pore radii which is shifted towards pores with a relatively small radius (more than 50% of pores have radius smaller than 800 .ANG.).